More particularly, the invention relates to a vehicle seat comprising a rigid support structure, a seat proper, a seat back provided with a rigid framework carried by the support structure, and a headrest, the framework of the seat back being connected to the support structure via at least first and second couplings that are disposed at different heights and that are adapted:                normally, to hold the seat back in an in-use position; and        when the seat back is subjected to high rearward stress, greater than a predetermined limit, to enable at least a top portion of the seat back to move rearwards at least to a safety position.        
Document U.S. Pat. No. 6,109,692 gives an example of such a seat that suffers from the drawback of having a voluminous seat back.